Combined gas chromatography mass spectrometry (GC/MS) is a well established analytical technique. Typically, injection volumes of a few microliters into the inlet of a gas chromatograph are analyzed from extracted samples. Since routine detection levels are on the order of one picogram, the total range in analyte concentrations delivered to an ion source can vary by >109. Since the dynamic range of modem instruments are several orders lower than this, premature electron multiplier failure, source sensitivity loss and quadrupole contamination can occur due to excessive sample loading. Historically, pre-screening of sample extracts utilizing a flame ionization detector or other means have been used to determine appropriate dilution factors. Dilution of sample extracts can bring concentrated analytes within the working range of the mass spectrometer as well as serve to protect it from premature degradation of sensitivity, resolution and tune. While this method is effective in providing longer service intervals for GC/MS instrumentation, it suffers from imposing a reduced sensitivity for all components of interest even during “clean” areas of a chromatogram. In addition, this requires extending the degree of sample handling and preparation.
Referring to FIG. 1, a typical prior art quadrupole mass spectrometer is illustrated. A filament 17 powered by filament supply 20 emits electrons which are accelerated toward a grounded ion volume 11. Since the filament is biased by a voltage source 19, electrons gain kinetic energy as they travel toward the ion volume and subsequently ionize a portion of sample molecules existing within the confines of the ion volume. These ions are extracted and focused in a continuous manner by a set of lens elements 14, 15, and 16 and are drawn into a quadrupole mass filter 10 which is biased at a suitable potential to give a predetermined ion energy. RF and DC potentials applied to the rods of the mass filter allow for selective mass transmission to a suitable detector 12. In this prior art method, electrons are emitted continuously by the filament 17 and can be measured by a sensor 18. This information can be fed to the filament supply 20 to control the filament temperature and thereby provide current regulation based on total emission current. This prior art method employs the use of a continuous beam ion source coupled to a continuous beam mass analyzer in which electrons flow continuously into an ion volume.
Referring to FIG. 2, a typical ion trap mass spectrometer is illustrated. In this type of mass analyzer, a non continuous beam ion source 32 is coupled to a non continuous beam mass analyzer defined by a single ring electrode 30 and a pair of endcap electrodes 31. A filament 38 powered by filament supply 41 emits electrons which are accelerated toward a grounded ion volume 32. Since the filament is biased by a voltage source 40, electrons gain kinetic energy as they travel toward the ion volume and subsequently ionize a portion of sample molecules within the ion volume. In these devices, it is necessary to introduce ions into a trapping field prior to mass analysis. The formation of ions or their injection into the trapping field must be done in an inject then scan fashion consistent with this batch mode of mass analysis. These ions are extracted from the ion source and focused in a non-continuous pulsed mode into the trap, by applying an extraction waveform to a gate electrode 36. Pulsed ion beams have been required in ion trapping devices due to the non-continuous nature of mass analysis. Though pulsing of the ion beam resolves the requirement for inject then scan, it has been found that excessive neutral noise from metastable helium atoms results if the filament emits electrons into the ion volume during the scan out of ions. For this reason, it is generally desired to reduce the electron energy below that required for metastable atom formation, or to stop the electron current into the ion volume entirely when mass analysis occurs. This problem has been addressed as described in U.S. Pat. No. 5,756,996 and in a modified version in U.S. Pat. No. 6,294,780. Operation of ion trap mass spectrometers is described in U.S. Pat. Nos. 4,540,884 and 4,736,101.
One disadvantage of ion trapping devices is that they suffer from space charge limitations of the number of ions which can be stored in the trap. Consequently, it is necessary to alter the ion injection time or ion formation time using automatic gain control (AGC), in order to reduce the population of ions in the trap and prevent these space charge saturation effects from occurring. It has been noted on these devices that since the total number of ions delivered to the mass analyzer and subsequently the detector are limited, that increased multiplier lifetime and analyzer cleanliness are maintained. It has also been observed that switching off the electron flow into the ion volume completely during non-injection as described in Wells et al. (U.S. Pat. No. 6,294,780), or by reducing the electron energy to a level which gives poor ionization efficiencies during non-injection as in Bier et al. (U.S. Pat. No. 5,756,996) also helps to keep ionizer components clean.
While AGC can be used in ion trapping devices to control total ion populations within the trap, doing so reduces their abundances in equal proportions. This has the deleterious effect of precluding detection of small abundance ions in the presence of large ion currents.